多表更新不能没有EXISTS

UPDATE dictionary_distributor a
SET a.name =
(SELECT b.name
FROM jxs_test b
WHERE a.code = b.oldcode)
WHERE EXISTS
(SELECT 1
FROM jxs_test b
WHERE a.code = b.oldcode);

UPDATE dictionary_distributor a
SET a.code =
(SELECT b.newcode
FROM jxs_test b
WHERE a.code = b.oldcode)
WHERE EXISTS
(SELECT 1
FROM jxs_test b
WHERE a.code = b.oldcode;